Interstellar Amorphous Silicates: Prolate vs. Oblate? Porous vs. …lana/DUST2016/Program_files/... · Are interstellar grains fairly smooth and compact? Presolar onion-like graphite

Interstellar Amorphous Silicates:

Prolate vs. Oblate?

Porous vs. Dense?

How Much of the Opacity is in a Separate Dust Component?

B. T. Draine

Princeton University

Brandon S. Hensley

Jet Propulsion Laboratory

1 B.T. Draine Interstellar Amorphous Silicates Garching 2016.09.13

Interstellar Silicates

• 1968: Discovery of emission feature near10µm (Gillett et al. 1968)• 1969: Identification as Si-O stretching mode in

silicates: (Woolf & Ney 1969; Gilman 1969;Hoyle & Wickramasinghe 1969)• 1969: 10µm feature seen in ISM absorption

(Knacke et al. 1969): silicates are ubiquitous• Silicates in molecular clouds appear to differ

from silicates in diffuse ISM (e.g., van Breemenet al. 2011)

• Extinction Profile in Diffuse ISM:– Strength:

Much of interstellar Si must be in silicates– Profile:

Predominantly amorphous (noncrystalline)exact composition uncertain

• 10µm feature polarized: grains are nonsphericaland aligned• polarization profile 6= extinction profile

solid curve: extinction profile(Fogerty et al. 2016; Poteetet al. 2016)

symbols: polarization profile(Wright et al. 2002)


Grain Geometry: Unknown

• Are interstellar grains fairly smooth and compact?

Presolar onion-like graphite grain (diameter ∼5µm ). Photo from S. Amari.

• Or are they typically loose aggregates of smaller particles, with a large “porosity”?

Two interplanetary dust particles collected from stratosphere (diameter ∼10µm ).Elemental compositions similar to primitive meteorites: silicates + carbonaceous

material.Images courtesy E.K. Jessberger and Don Brownlee.


Grain Composition: “Pure” or “Mixed”?

• Substantial fraction of inter-stellar grain mass consists ofamorphous silicate material• All grain models include other

materials– Carbonaceous material– Metallic iron?– Iron oxides?

Are different grain materials seg-regated into separate grains (e.g.,silicate grains and carbonaceousgrains)?

Or are they mixed together in composite grains?

Two interplanetary dust particles collected fromstratosphere (diameter ∼10µm ).

Elemental compositions similar to primitivemeteorites: silicates + carbonaceous material.

Images courtesy E.K. Jessberger and DonBrownlee.


Theory: How Can We Determine Grain Geometry?Optics of small particles

Polarization profile depends on• shape of silicate grains• strength of absorption

Can infer shape from observed 10µmpolarization profile (Martin 1975).

Early work:• Martin (1975): either

– oblate (b/a > 1.5) orprolate (a/b > 2.5)

– ∆Cabs(10µm )/V ≈ 3000 cm−1

• Draine & Lee (1984):– ∼2:1 oblate– ∆Cabs(10µm )/V ≈ 104 cm−1

Two approaches• Could seek mix of lab materials that can

reproduce extinction and polarization withsuitable choice of shape.Difficulties:– may not have “right” materials in lab– if more than one material, how to

“mix”?use effective medium theory?if so, which EMT...?

• Synthetic “effective” dielectric function:– Assume some shape– “solve” for dielectric function ε(λ) that

reproduces extinction– calculate polarization for model– compare model to observed polarization– identify shape that gives best agreement

with observed polarization


Plan of Attack

• Obtain best available determination of silicate extinction profile• Design dielectric function to exactly reproduce this profile

for assumed silicate– shape– porosity– Additional degree of freedom: strength of “continuum” absorption associated with

the silicate-bearing grains

• Calculate polarization profile for the model• Compare model to observed polarization• Find shape and porosity and continuum absorption that result in best match to observed

polarization.


ObservationsExtinction

Cyg OB2-12 (` = 80◦, D ≈ 1.7 kpc)(Fogerty et al. 2016; Poteet et al. 2016)

GC = sightline to Galactic Center(Kemper et al. 2004)

Polarization

WR55 (` = 308◦, D ≈ 3.5 kpc) andWR112 (` = 12◦, D ≈ 4.1 kpc)

(Wright et al. 2002)

somewhat shocking note:these data taken on AAT in 1992


ModelDielectric Function ε(λ)

• based on “astrosil” absorption at λ < 1µmplus

• sum of 3000 Lorentz oscillators distributedbetween λ = 1µm and λ = 3 cm, each con-tributing a strongly-peaked absorption pro-file.• Assume shape (spheroid or continuous

distribution of ellipsoids)• Assume strength

∆αsil ≡ ∆Cabs(9.7µm )/V

• Assume f8 = fraction of 8µm extinctioncontributed by silicate-bearing grains• Reproduce “observed” absorption:

– “observed” silicate profile (λ = 1µm −30µm )

– FIR and submm (30µm − 1 cm) consis-tent with Planck

• Iteratively adjust “strengths” of the N =3000 Lorentz oscillators.

Shapes• Spheroids (oblate and prolate)• Continuous distributions of ellipsoids

– CDE from Bohren & Huffman(1983) is unrealistic – includes veryextreme shapes

– ERCDE = “Externally RestrictedCDE” (Zubko et al. 1996) excludesvery extreme shapes (sharp cutoff)

– CDE2 = smooth suppression ofextreme shapes (Ossenkopf et al.1992)


Distribution of “Shape Factors” LjFor ~E ‖ principal axis j:

Cabs

V=

4πω

cIm

[ε− 1

(ε− 1)Lj + 1

]

• g(L)dL = probability of “shapefactor” in dL

• Ellipsoids: L1 + L2 + L3 = 1(and Lj > 0)

• ~E ‖ needle: Lj = 0

– No “depolarization”– Maximum absorption.

• ~E ⊥ plate: Lj = 1

– Maximum “depolarization”– Minimum absorption


Silicate Absorption “Strength”

• Silicate extinction is strong:∆τ9.7µmAV

≈ 0.051± 0.003 mag−1

• If ∼100% of interstellar Si is in amor-phous silicate material then

∆κ9.7µm = 2770 cm2 g−1

• Let fsil ≡ fraction of “volume” ofsilicate-bearing grains that is occupiedby silicate material with ρ = 3.4 g cm−3

Then

αsil ≡∆C9.7µm

V= 9410 cm−1 × fsil

• fsil = 1− (porosity + fraction of volumeoccupied by non-silicate material)

f8

f8 ≡ fraction of 8µm extinction con-tributed by silicate-bearing grains.

• If only one grain type (agglomerationsof silicate and other material)then f8 = 1.

• Polarization allows estimation of f810 B.T. Draine Interstellar Amorphous Silicates Garching 2016.09.13

Dielectric FunctionsFor each choice of shape, ∆αsil, and f8:

ε(λ) = ε1 + iε2

Shape matters!More extreme shape→ smaller ε2→ smaller ε1 at long λ


Polarization Profiles: Dependence on Shape

More extreme shape→ larger Cpol/V

Normalized profile also depends onshape.


Polarization Profiles: Dependence on Strength ∆αsil ≡ ∆Cabs/V

Stronger absorption→ larger Cpol/VNormalized profile also depends on ∆αsil (although for oblate shapes, shift is small)


Shape and Strength

• χ2 = goodness-of-fit metric(smaller is better)

• Best fit for oblate shapes (b/a >∼ 2)

• CDE2 shape distribution gives fitsimilar to b/a = 2 oblate spheroids

• Fit is better for strong absorption:

∆αsil>∼ 6000 cm−1

• Porosity <∼ 1− (6000/9410) ≈ 0.35

silicate-bearing grainsare not highly porous


Contribution of Silicate-Bearing Grains to 8µm Extinction

• Best fit: f8 ≈ 0.2± 0.2

• Implication:

Most of the 8µm extinctionis provided by grains that are

not silicate-bearingi.e., a separate grain population


Polarization of Far-Infrared Emission

• If CDE2 grains (or b/a ≈ 2 grains)were perfectly-aligned, FIR emission∼60% polarized!!

• Observed starlight polarization:if b/a ≈ 2 oblate spheroids

→ silicate grain alignment falign ≈ 0.35

→ up to ∼20% polarization in FIR.

• Planck: very small fraction of sight-lines have FIR polarization p > 20%.


SUMMARY

• Silicate-bearing grains in the diffuse ISM:

∼2:1 oblate (or ∼ CDE2 shape distribution)

Low porosity: porosity <∼ 35%

Account for 0.05 <∼ f8<∼ 0.4 of the observed extinction at 8µm :

A separate grain component (carbonaceous grains?) accounts formost of the observed extinction at 8µm

• Self-consistent dielectric function appears consistent with– Starlight polarization in optical– 10µm silicate polarization– FIR-submm polarized emission

• To more strongly constrain physical grain models: Need accurate mea-surements of polarization in 7.9−13.4µm and 15.5−21.0µm windows

CanariCam on GTC?... New ESO instrument?


THANK YOU


REFERENCES REFERENCES

ReferencesBohren, C. F., & Huffman, D. R. 1983, Absorption and Scattering of Light by

Small Particles (New York: Wiley)

Draine, B. T., & Lee, H. M. 1984, Ap. J., 285, 89

Fogerty, S., Forrest, W., Watson, D. M., Sargent, B. A., & Koch, I. 2016,arXiv:1608.06987

Gillett, F. C., Low, F. J., & Stein, W. A. 1968, Ap. J., 154, 677

Gilman, R. C. 1969, Ap. J. Lett., 155, L185

Hoyle, F., & Wickramasinghe, N. C. 1969, Nature, 223, 459

Kemper, F., Vriend, W. J., & Tielens, A. G. G. M. 2004, Ap. J., 609, 826

Knacke, R. F., Gaustad, J. E., Gillett, F. C., & Stein, W. A. 1969, Ap. J. Lett.,155, L189

Martin, P. G. 1975, Ap. J., 202, 393

Ossenkopf, V., Henning, T., & Mathis, J. S. 1992, Astr. Ap., 261, 567

Poteet, C. P., Chiar, J. E., & Whittet, D. C. B. 2016, in preparation, 000, 000

van Breemen, J. M., et al. 2011, Astr. Ap., 526, A152

Woolf, N. J., & Ney, E. P. 1969, Ap. J. Lett., 155, L181

Wright, C. M., Aitken, D. K., Smith, C. H., Roche, P. F., & Laureijs, R. J.2002, in The Origin of Stars and Planets: The VLT View, ed. J. F. Alves &M. J. McCaughrean, 85

Zubko, V. G., Mennella, V., Colangeli, L., & Bussoletti, E. 1996, M.N.R.A.S.,282, 1321


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Interstellar Amorphous Silicates: Prolate vs. Oblate? Porous vs. …lana/DUST2016/Program_files/... · Are interstellar grains fairly smooth and compact? Presolar onion-like graphite